The present invention relates to a method of manufacturing a metal carcass for a flexible pipe or umbilical and relates more particularly to the manufacture of a metal strip that can be used to produce a metal carcass.
The flexible pipe to which reference is made in what follows is the pipe used in off-shore oil production and which comprises, from the inside outwards, a metal carcass, an internal sealing sheath, a pressure vault, one or more layers of armour and an external sealing sheath. A flexible pipe of this kind is known to the specialists as a rough-bore flexible pipe as opposed to a so-called smooth-bore flexible pipe wherein the innermost element consists of an internal sealing sheath on which a pressure vault is wound.
These flexible pipes have to exhibit a certain number of mechanical properties particularly collapse strength to allow them to resist collapse which may be brought about by the external pressure, when the flexible pipe is in service, particularly in deep-water applications, and/or the contact pressures exerted on the flexible pipes, particularly during laying and handling. Now, this collapse strength often depends on the mechanical integrity of the metal carcass which, practically alone, has to withstand the force of the external pressure.
The metal carcass is generally made of a profiled strip wound into interlocked turns, for example, an interlocked strip or interlocked shaped wire such as a T-shaped, U-shaped, S-shaped or Z-shaped wire. The shaped strip or wire is wound at an angle in the region of 90.degree., so that the flexibility of the metal carcass produced is not affected, so as to give the flexible pipe comprising such a metal carcass sufficient flexibility.
This being the case, there are a number of solutions which have been proposed to attempt to improve the collapse strength.
A first solution consists in increasing the thickness of the strip used to produce the metal carcass, but keeping the same S-shaped profile. However, there is an upper limit of the thickness which cannot be exceeded, and of the order of 3 mm. Above this thickness, shaping tools would no longer be able to shape the strip, unless extremely robust tools were made, but this is not economically acceptable. Furthermore, the increase in pipeline weight is not compatible with deep-water applications.
A second solution consists in increasing the height of its carcass while keeping the same profile and the same initial strip thickness. This kind of solution produces a carcass whose weight is equivalent to that of the first solution. This type of carcass is described in EP-0,429,357. Here too, the thickness of the strip which cannot exceed 3 mm proves to be a limiting factor.
A third solution consists in increasing the second moment of area of the profile by simplifying it; to this end, a carcass of the so-called box-section strip has been employed. However, the box-section strip led to an increase in the thickness of the strip and in the volume of the metal carcass and therefore to an increase in the weight of the flexible pipe and in its cost. The box-structure carcass is described in EP-0,494,299.
A fourth solution consists in increasing the yield strength of the strip by using high performance metal alloys such as duplex stainless steels or any other alloy with good mechanical properties (nickel-based alloys). This last solution considerably increases the cost of the metal carcass and therefore of the flexible pipe for certain uses thereof.
A flexible-pipe carcass may deform when a significant load is applied to it, the formation being in two main modes, namely the cardioid mode or the ovalized mode. To prevent ovalization of the carcass from occurring too rapidly, a pressure vault may be used which is dimensioned such that ovalization is delayed for as long as possible. Thus, in practice, and under the conditions of use of the flexible pipe, the metal carcass tends to deform only in cardioid mode.
To improve the performance (collapse strength) of the metal carcass in cardioid mode, it is necessary to improve the mechanical properties of the materials used for the carcass. However, when the mechanical properties of the strip from which the metal carcass is made are improved in a controlled manner, this sometimes results in a strip which can no longer be shaped to a determined profile because it has an insufficient elongation at rupture (A%).